Orientation dependent etching (ODE) is a wet etching step which attacks different crystalline planes at different rates. As is well known in the art of orientation dependent etching, etchants such as potassium hydroxide, or TMAH (tetramethylammonium hydroxide), or EDP etch the (111) planes of silicon much slower (on the order of 100 times slower) than they etch other planes. A well-known case of interest, described in U.S. Pat. No. 3,765,969, is the etching of a monocrystalline silicon wafer having (100) orientation. There are four different orientations of (111) planes which intersect a given (100) plane. The intersection of a (111) plane and a (100) plane is a line in a [110] type direction. There are two different [110] directions contained within a (100) plane. They are denoted as [011] and [01-1] and are perpendicular to one another. Thus, if a monocrystalline silicon substrate having (100) orientation is covered with a layer, such as oxide or nitride which is resistant to etching by KOH or TMAH, but is patterned to expose a rectangle of bare silicon, where the sides of the rectangles are parallel to [110] type directions, and the substrate is exposed to an etchant such as KOH or TMAH, then a pit will be etched in the exposed silicon rectangle. If the etch is allowed to proceed to completion, then the pit will have four sloping sides, each side being a different (111) plane. Because the (111) planes etch so slowly, the process is said to be self-terminating. The shape and dimensions of the pit are very predictable and reproducible, being relatively insensitive to the etch bath conditions or etching duration, as long as the etching has been allowed to proceed to completion. If the length and width of the rectangle of exposed silicon were L and W respectively, and if L=W, then the four (111) planes would meet at a point, and the pit would be pyramid shaped. The (111) planes are at a 54.7 degree angle with respect to the (100) surface. The depth H of the pit is half the square root of 2 times the width, that is, H=0.707 W. If L>W, then the maximum depth H is still 0.707 W and the shape of the pit is a V groove with sloped sides and sloped ends. The length of the region of maximum depth of the pit is L−W. Of course, if the thickness of the substrate is less than 0.707 W, and if the etch is allowed to proceed to completion, then a hole will be etched through the substrate.
One constraint of orientation dependent etching of self-terminated pits in (100) wafers is that, if etched to completion, they will intersect the wafer surface as a rectangle whose sides are parallel to [110] type directions. Arbitrary shapes are not allowed. FIG. 1A is a top view of a self-terminated orientation dependent etched pit 11 having length L and width W in a (100) wafer. Region 12 has been covered by masking layer, such as an oxide or a nitride, so that the (100) wafer surface was not exposed to the etchant. Region 13 is a rectangle with sides parallel to [110] directions. In region 13, the masking layer was removed prior to orientation dependent etching, so that the wafer surface was exposed. FIG. 1B is a cross-section of rectangular pyramid shaped pit 11 through line 1B-1B. Maximum depth of pit 11 is H=0.707 W.
FIG. 2 shows one example of what occurs if the exposed region 23 is not a rectangle with sides parallel to [110] type directions. As seen in the top view of FIG. 2A, all sides of the exposed region are parallel to [110] type directions, but the exposed region 23 has an abrupt change in width from W1 to W2, as if a wide rectangle having length L1 and a narrow rectangle having length L2 had been exposed end to end. Stated in another way, the exposed region 23 is a polygon with at least one convex corner 24. A convex corner is defined here as a region which bulges into the polygon. A convex corner has the property that if a line is drawn between adjacent sides of the corner, the line will lie outside the polygon. Line 25 in FIG. 2A is an example. There are two convex corners in FIG. 2A, but only convex corner 24 is labeled. FIG. 2B shows a top view of the resulting pit 21 if etched to completion. The masking layer has been removed for greater visibility of the etched pit 21. Etching continues at a rapid rate even under the masking layer 22, until the final shape is a rectangular pyramid having width W1, length L1+L2, maximum depth H=0.707 W1, and no convex corners.
FIG. 3 shows a second example of what occurs if the exposed region is not a rectangle. In this case, the exposed region 33 consists of two rectangles, each having sides parallel to [110] type directions, which intersect in a T. Exposed region 33 has two convex corners, one of which is labeled as 36. Line 37 is drawn between adjacent sides to the convex corner and lies outside exposed region 33. The length and width of rectangle 34 are L1 and W1, and the length and width of rectangle 35 are L2 and W2, where L2>L1. FIG. 3B shows a top view of the resulting pit 31 if etched to completion. Etching will continue at a rapid rate even under the masking layer 32 until the final shape is a rectangular pyramid having length W1+L2, width L1, maximum depth H=0.707 L1, and no convex corners.
Because of the precision and reproducibility of orientation dependent etched features in (100) wafers, a variety of applications have been developed. One family of applications is related to the formation of fluid passageways, including fluid inlet holes, fluid filters, fluid manifolds, fluid flow restrictors, and individual fluid channels. It is frequently desired to join one or more of such fluid passageway components in a fluidic device, such as an ink jet printhead. However, due to the constraints of orientation dependent etching described above, such different components typically cannot be joined together by means of orientation dependent etching to completion.
U.S. Pat. No. 4,601,777 discusses various processes for fabricating thermal ink jet printheads. FIG. 4 shows a top view of a group of ink channels 41 which are desired to be fluidically connected to ink manifold 42. In this case the V-shaped grooves which will comprise channels 41 are formed by a self-terminating orientation dependent etching process, which is preferred because it is desired to precisely control the channel dimensions. The ink manifold 42 is formed by a timed orientation dependent etching process. The grooves forming the channels are formed close to the manifold, but not connected to it in the initial etching process. A narrow region 43 initially isolates the channel grooves from the manifold. Two alternatives are disclosed for making fluidic connection between the manifold 42 and the channels 41. The first alternative is to isotropically etch to undercut the nitride mask in the narrow isolation region 43, followed by a brief orientation dependent etch to complete the opening of the channels to the manifold. A disadvantage of this approach is that during the timed orientation dependent etch to join the channels to the manifold, the walls 44 between channels 41 nearest to the ends of the channels closest to the manifold 42 etch at a rapid rate, so that the precision and reproducibility of the channel dimensions are compromised somewhat. A second alternative described by U.S. Pat. No. 4,601,777 is to remove the narrow region 43 by a subsequent dicing operation. A disadvantage of this alternative, which is disclosed in the patent, is that the dicing operation also removes material which is not desired to be removed and which must be replaced in a subsequent sealing operation.
A second configuration of joining of fluidic passageways formed by orientation dependent etching is described in U.S. Pat. No. 4,639,748. In this case it is desired to join an orientation dependent etched fluid manifold to a particle filter comprised of a pattern of recesses which have been orientation dependent etched. The method of making the connection is to use an isotropic etch followed by an orientation dependent etch, similar to the first alternative described above for U.S. Pat. No. 4,601,777.
A third instance of joining of fluidic passageways formed by orientation dependent etching is described in U.S. Pat. No. 4,774,530. In this case it is desired to connect ink jet channels to an ink manifold. The channels and manifold are etched in an upper substrate with is aligned and mated to a lower substrate. On the lower substrate is a thick film layer which is patterned in such a way that fluidic connection is made between the channels and manifold. Such a thick film layer, however, is not always available in devices where it is desired to make passageways to connect orientation dependent etched features.
In addition to the forming of fluidic passageways, orientation dependent etched features are also used various other different types of applications. For example, the capability of forming precision V grooves by orientation dependent etching has been frequently used as a means for precision alignment of optical components, such as the end-to-end alignment of optical fibers, or the alignment of a laser to optical fibers.
Furthermore, orientation dependent etched features have been used in processes for fabrication of integrated circuit components, for example providing electrical isolation while minimizing parasitic capacitance (U.S. Pat. No. 4,685,198).
Orientation dependent etching is also frequently used in fabrication of a variety of microelectromechanical systems (or MEMS) devices.
Recognizing that orientation dependent etching has a wide range of applications, and that methods are desirable for forming a passageway or recess which is connected to one or more orientation dependent etched feature, this invention is directed toward such methods.